Chapter 10: Carbohydrates

Carbohydrates are essential carbon-based biomolecules, primarily composed of carbon, hydrogen, and oxygen, rich in hydroxyl groups. These molecules serve various functions in living organisms, including energy storage, structural support, and cellular recognition processes.

Common empirical formula: (CH₂O)n, which reflects the hydrate nature of carbohydrates, with n representing the number of carbohydrate units.

Monosaccharides: These are the simplest forms of carbohydrates, consisting of single sugar molecules that can exist in both linear and ring forms. Examples of monosaccharides include:

  • Glucose: A six-carbon aldehyde (aldohexose) that is a primary energy source for cells.

  • Fructose: A six-carbon ketone (ketohexose) found in many fruits.

  • Galactose: An aldohexose that is part of lactose.

Monosaccharides are categorized by the number of carbon atoms they possess:

  • Trioses: 3 carbons (e.g., glyceraldehyde)

  • Tetroses: 4 carbons (e.g., erythrose)

  • Pentoses: 5 carbons (e.g., ribose, which is vital for RNA)

  • Hexoses: 6 carbons (e.g., glucose, fructose, galactose)

  • Heptoses: 7 carbons (e.g., sedoheptulose)

These compounds can be represented structurally using Fischer projections, which highlight their configuration and stereochemistry. Here’s a diagram of the Fischer projection for glucose:

       H        OH
        |        |
    H – C – C – C – C – OH
        |        |        |
       OH     H      H
        |         |      |
       H      H       OH

Oligosaccharides and Polysaccharides: Oligosaccharides are formed by the condensation of two or more monosaccharides linked together by O-glycosidic bonds. Common disaccharides include:

  • Maltose: Comprising two glucose units linked by a extα−1,4ext{α-1,4} glycosidic bond, maltose is produced during the digestion of starch.

  • Sucrose: Consisting of glucose and fructose linked by a extβ−1,2ext{β-1,2} bond, sucrose is commonly known as table sugar and serves as an energy source.

  • Lactose: A disaccharide composed of glucose and galactose linked by a extβ−1,4ext{β-1,4} bond, lactose is the sugar found in milk.

Polysaccharides, formed from many monosaccharide units, serve critical roles in energy storage and structural integrity. Noteworthy examples include:

  • Glycogen: A highly branched polysaccharide used for energy storage in animals, predominantly in the liver and muscle.

  • Starch: A primary storage form of energy in plants, available in both branched (amylopectin) and unbranched (amylose) forms.

Diagram of Glycogen Structure:

       Glucose
         |   
      Glucose -- Glucose
         |            | 
      Glucose     Glucose
         |            | 
       ...         ...

Glycoproteins: These molecules are composed of carbohydrates covalently attached to proteins, constituting a significant portion of the proteome, approximately 50%. Glycoproteins serve multiple functions, including:

  • Cell Adhesion: Promoting cell-cell interactions essential for tissue formation.

  • Immune Response: Involvement in the recognition of pathogens.

Classifications of glycoproteins include:

  • Glycoproteins: Protein predominates by weight and is involved in various biological functions.

  • Proteoglycans: These are proteins modified with glycosaminoglycans, skewing the carbohydrate content, which predominantly serves structural roles in the extracellular matrix.

  • Mucins: Highly glycosylated proteins that serve as lubricants in mucus, playing vital roles in protecting epithelial surfaces.

Diagram of Glycosylation Process:

 Carbohydrate 
    |  
Protein + Sugars ---> Glycoprotein
    |  
Sugars can link to Asparagine (N-linkage) or Serine/Threonine (O-linkage)

Lectins: These are specialized carbohydrate-binding proteins that facilitate critical interactions between cells and their environment, playing essential roles in cellular signaling and immune responses. By binding to specific carbohydrates, lectins can mediate cell-cell interactions and influence diverse biological processes, such as pathogen recognition and cell adhesion.

Monosaccharides and Isomerism: Monosaccharides can exhibit a range of structural isomers—compounds with the same molecular formula but different arrangements of atoms:

  • Structural Isomers: Differ in the connectivity of their atoms (e.g., glucose and fructose).

  • Stereoisomers: Mirror images of each other, such as D- and L-forms.

  • Epimers: Sugars differing at one specific asymmetric carbon, e.g., glucose and mannose.

The formation of ring structures occurs through hemiacetal or hemiketal formation in solution, which significantly impacts their biological functions. Common cyclic forms include:

  • Pyranose: A six-membered ring, prevalent in glucose.

  • Furanose: A five-membered ring structure found in fructose.

Diagram of Pyranose and Furanose Forms:

   Pyranose (Glucose)
         O 
        / \ 
       C1--C6 
      /        
    C2 + C5   
      \        
       C3--C4

   Furanose (Fructose)
         O 
        / \ 
     C2-- C5 
      |      |  
     C3  C4

Chemical Properties: Monosaccharides are chemically versatile and can participate in various reactions:

  • Modifications: Can undergo chemical reactions to produce derivatives like glycosides and nucleosides, which enhance their biochemical functions.

  • Reducing Sugars: Many monosaccharides have the capability to reduce cupric ions, which is utilized in clinical settings for diagnostic tests.

Clinical Relevance: Carbohydrates are vital in metabolism, with particular emphasis on glucose homeostasis. For example, glycated hemoglobin (HbA1c) is an important clinical marker for monitoring long-term glucose levels in diabetes management. Furthermore, genetic disorders affecting glycosylation, such as I-cell disease, can lead to significant pathologies and underscore the importance of carbohydrates in health and disease management.